Trends in ammonia emissions from light-duty gasoline vehicles in China, 1999-2017.

Ammonia (NH) emitted from motor vehicles is a by-product of measures taken to reduce emissions of other pollutants (e.g. NOx and CO) and has potentially important environmental impacts. NH levels can be impacted by various emission standards. However, there is a lack of investigations of the influences from the implementation of different vehicular emission standards on long-term changes in NH emissions. To fill this gap, we estimated the inter-annual NH emissions of light-duty gasoline vehicles (LDGVs) under different emission standards (State 0 to State V) from 1999 to 2017 and investigated the emission change characteristics under the rapidly developing Chinese economy. Results showed that the NH emissions from LDGVs had a sharp, 42-fold increase (from 1.8Gg to 77.9Gg). However, NH emissions per capita have begun to decrease with increases in socioeconomic development, presenting an inverted U-shaped tendency as a function of per capita GDP. Further exploration indicated that the decline in emission factors, as determined by upgrades in emission standards, was the decisive factor in promoting the downward trend in per capita emissions. This suggests that continuously upgrading emission standards has offset the increase in NH emissions due to the rapid growth of motor vehicles. Quantitative scenario analysis showed a two-stage impact of emission standards on NH emissions: emissions would decrease 77% (48%-90% for different years) if State I and State II were not implemented; while if none of standards were upgraded (State III to State V), NH emissions would increase 118% (13%-224% for different years), 2-6 times the impacts from the growth of vehicle population and the decline of vehicle kilometres traveled. The data and findings in this study can provide scientific support for understanding air pollution in urban areas and for formulating further vehicle pollution mitigation measures in China and other countries.